Highway Engineering

the month within which the limited survey is collated, and are based on an exten-
sive range of surveys undertaken over the past 40 years by the Transport
Research Laboratory.
E-factors are used to transform 12-hour flows into 16-hour flows. M-factors
are utilised to transform 16-hour flows into AADT values. These factors are
supplied in Tables 4.20 and 4.21 (DoT, 1996).
For the purposes of this conversion process, highways are categorised as:

Motorways

Built-up trunk roads (speed limit £40 mph)

Built-up principal roads (speed limit £40 mph)

Non built-up trunk roads (speed limit >40 mph)

Non built-up principal roads (speed limit >40 mph).

The above network classification is utilised to call default valuations for sea-
sonality index (SI) as well as for the E-factors. SI is defined as the ratio of the
average August weekday flow to the average weekday flow in the so-called
neutral months (April, May, June, September and October). The ‘neutral’
months are deemed so because they are seen as being the most representative
months available for extrapolation into full-year figures.
As seen in Table 4.19, the SI value defines the M-factor (a value that can be
overwritten if local information is available). A good estimate of SI is derived
by comparison of the weekday (Monday to Friday) flows of three-week con-
tinuous traffic counts from the month of August with those of late
May/June/October.
General ranges and typical default values of seasonality index for the
different classes of highway are shown in Table 4.19.

Basic Elements of Highway Traffic Analysis 99

Network classification Range of SI Typical SI

valuation

Motorway (MWY) 0.95–1.35 1.06

Built-up trunk roads (TBU) 0.95–1.10 1.00

Built-up principal roads (PBU) 0.95–1.15 1.00

Non-built-up trunk roads (TNB) 1.00–1.50 1.10

Non-built-up principal roads (PNB) 1.00–1.40 1.10

Table 4.19Default SI

values for each highway

classification

(DoT, 1996)

At a minimum, a 12-hour count (0700 to 1900) must be available. It
should preferably be for one of the five neutral months, as traffic models
not based on these are considered less reliable in terms of establishing annual
flows. In the absence of local information, default E-factors for converting
the 12-hour counts to 16-hour equivalents (0600 to 2200) are as shown in Table
4.20.